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Digestive System and Nutrition Chapter Concepts 12.1 The Digestive System • The human digestive system is an extended tube with specialized parts between two openings, the mouth and the anus. 214 • Food is ingested and then digested to small molecules that are absorbed. Indigestible remains are eliminated. 214 le b ila a v A ot N “ 12.2 Three Accessory Organs • The pancreas, the liver, and the gallbladder are accessory organs of digestion because their activities assist the digestive process. 222 12.3 Digestive Enzymes • The products of digestion are small molecules, such as amino acids and glucose, that can cross plasma membranes. 224 • The digestive enzymes are specific and have an optimum temperature and pH at which they function. 225 in t x Te e- n” o si r Ve 12.4 Nutrition • Proper nutrition supplies the body with energy and nutrients, including the essential amino acids and fatty acids, and all vitamins and minerals. 226 Scanning electron micrograph of the stomach shows how the wall is folded for easy expansion. There are many openings to gastric pits, where hydrochloric acid and an enzyme for protein digestion are produced. © R. G. Kessel and R. H. Kardon, “Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy”. W. H. Freeman, 1979, all rights reserved. 213 Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 214 Part 3 12-2 Maintenance of the Human Body njoying the summer night at an outdoor cafe, Evora washes down her last piece of garlic bread with a sip of wine. Even before Evora swallowed the bread, her mouth’s saliva began to break it apart. The wine’s alcohol is absorbed in the stomach, where the process of transforming Evora’s meal into a nutrient-laden liquid begins. In the small intestine, wormlike projections from the intestinal wall absorb amino acids, sugars, and other needed molecules into Evora’s bloodstream. Even the large intestine contributes by taking in needed water and salts. Her body now refueled, Evora heads off for a night of dancing. In this chapter, you will learn how the body digests food, and the importance of proper nutrition. Science is beginning to find the cellular basis for believing that fruits and vegetables, and yes, especially broccoli, can ensure a brighter and healthier future. The contents of this chapter should be of interest to everyone. E of muscle. The soft palate ends in a finger-shaped projection called the uvula. The tonsils are in the back of the mouth, on either side of the tongue and in the nasopharynx (called adenoids). The tonsils help protect the body against infections. If the tonsils become inflamed, the person has tonsillitis. The infection can spread to the middle ears. If tonsillitis recurs repeatedly, the tonsils may be surgically removed (called a tonsillectomy). Three pairs of salivary glands send juices (saliva) by way of ducts to the mouth. One pair of salivary glands lies at the sides of the face immediately below and in front of the ears. These glands swell when a person has the mumps, a viral View 1 12.1 The Digestive System M Digestion takes place within a tube called the digestive tract, which begins with the mouth and ends with the anus (Fig. 12.1). The functions of the digestive system are to ingest food, digest it to nutrients that can cross plasma membranes, absorb nutrients, and eliminate indigestible remains. View 2 pharynx oral cavity tongue salivary glands esophagus The Mouth The mouth, which receives food, is bounded externally by the lips and cheeks. The lips extend from the base of the nose to the start of the chin. The red portion of the lips is poorly keratinized and this allows blood to show through. Most people enjoy eating food largely because they like its texture and taste. Sensory receptors called taste buds occur primarily on the tongue, and when these are activated by the presence of food, nerve impulses travel by way of cranial nerves to the brain. The tongue is composed of skeletal muscle whose contraction changes the shape of the tongue. Muscles exterior to the tongue cause it to move about. A fold of mucous membrane on the underside of the tongue attaches it to the floor of the oral cavity. The roof of the mouth separates the nasal cavities from the oral cavity. The roof has two parts: an anterior (toward the front) hard palate and a posterior (toward the back) soft palate (Fig. 12.2a). The hard palate contains several bones, but the soft palate is composed entirely Back Forward Main Menu TOC diaphragm liver stomach gallbladder pancreas common bile duct pancreatic duct duodenum transverse colon small intestine ascending colon descending colon cecum sigmoid colon rectum anal canal appendix anus Figure 12.1 Digestive system. Trace the path of food from the mouth to the anus. The large intestine consists of the cecum; ascending, transverse, descending, and sigmoid colons; plus the rectum and anal canal. Note also the location of the accessory organs of digestion: the pancreas, the liver, and the gallbladder. Study Guide TOC Textbook Website Student OLC MHHE Website 12-3 Chapter 12 Digestive System and Nutrition 215 enamel crown dentin pulp hard palate gum soft palate root canal uvula tonsil root periodontal membrane molars (3) jawbone premolars (2) cementum canine (1) incisors (2) a. b. Figure 12.2 Adult mouth and teeth. a. The chisel-shaped incisors bite; the pointed canines tear; the fairly flat premolars grind; and the flattened molars crush food. The last molar, called a wisdom tooth, may fail to erupt, or if it does, it is sometimes crooked and useless. Often dentists recommend the extraction of the wisdom teeth. b. Longitudinal section of a tooth. The crown is the portion that projects above the gum line and is sometimes replaced by a dentist. When a root canal is done, the nerves are removed. When the periodontal membrane is inflamed, the teeth can loosen. infection most often seen in children. Salivary glands have ducts that open on the inner surface of the cheek at the location of the second upper molar. Another pair of salivary glands lies beneath the tongue, and still another pair lies beneath the floor of the oral cavity. The ducts from these salivary glands open under the tongue. You can locate the openings if you use your tongue to feel for small flaps on the inside of your cheek and under your tongue. Saliva contains an enzyme called salivary amylase that begins the process of digesting starch. The Teeth With our teeth we chew food into pieces convenient for swallowing. During the first two years of life, the smaller 20 deciduous, or baby, teeth appear. These are eventually replaced by 32 adult teeth (Fig. 12.2a). The third pair of molars, called the wisdom teeth, sometimes fail to erupt. If they push on the other teeth and/or cause pain, they can be removed by a dentist or oral surgeon. Each tooth has two main divisions, a crown and a root (Fig. 12.2b). The crown has a layer of enamel, an extremely hard outer covering of calcium compounds; dentin, a thick layer of bonelike material; and an inner pulp, which contains the nerves and the blood vessels. Dentin and pulp are also found in the root. Back Forward Main Menu TOC Study Guide TOC Tooth decay, called dental caries, or cavities, occurs when bacteria within the mouth metabolize sugar and give off acids, which erode teeth. Two measures can prevent tooth decay: eating a limited amount of sweets and daily brushing and flossing of teeth. Fluoride treatments, particularly in children, can make the enamel stronger and more resistant to decay. Gum disease is more apt to occur with aging. Inflammation of the gums (gingivitis) can spread to the periodontal membrane, which lines the tooth socket. A person then has periodontitis, characterized by a loss of bone and loosening of the teeth so that extensive dental work may be required. Stimulation of the gums in a manner advised by your dentist is helpful in controlling this condition. The tongue, which is composed of striated muscle and an outer layer of mucous membrane, mixes the chewed food with saliva. It then forms this mixture into a mass called a bolus in preparation for swallowing. The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue forms it into a bolus for swallowing. Textbook Website Student OLC MHHE Website 216 Part 3 12-4 Maintenance of the Human Body Table 12.1 Path of Food Organ Function of Organ Special Feature(s) Function of Special Feature(s) Oral cavity Receives food; starts digestion of starch Teeth Tongue Chewing of food Formation of bolus Esophagus Passageway Stomach Storage of food; acidity kills bacteria; starts digestion of protein Gastric glands Release gastric juices Small intestine Digestion of all foods; absorption of nutrients Intestinal glands Villi Release fluids Absorb nutrients Large intestine Absorption of water; storage of indigestible remains The Pharynx nasopharynx soft palate hard palate tonsil uvula tonsil bolus epiglottis covering glottis The pharynx is a region that receives food from the mouth and air from the nasal cavities (Table 12.1). The food passage and air passage cross in the pharynx because the trachea (windpipe) is ventral to (in front of) the esophagus, a long muscular tube that takes food to the stomach. Swallowing, a process that occurs in the pharynx (Fig. 12.3), is a reflex action performed automatically, without conscious thought. During swallowing, food normally enters the esophagus because the air passages are blocked. Unfortunately, we have all had the unpleasant experience of having food “go the wrong way.” The wrong way may be either into the nasal cavities or into the trachea. If it is the latter, coughing will most likely force the food up out of the trachea and into the pharynx again. Usually during swallowing, the soft palate moves back to close off the nasopharynx, and the trachea moves up under the epiglottis to cover the glottis. The glottis is the opening to the larynx (voice box). The up and down movement of the Adam’s apple, the front part of the larynx, is easy to observe when a person swallows. We do not breathe when we swallow. esophagus trachea Figure 12.3 Swallowing. When food is swallowed, the soft palate closes off the nasopharynx and the epiglottis covers the glottis, forcing the bolus to pass down the esophagus. Therefore, you do not breathe when swallowing. Back Forward Main Menu TOC Study Guide TOC The air passage and the food passage cross in the pharynx. When you swallow, the air passage usually is blocked off, and food must enter the esophagus. Textbook Website Student OLC MHHE Website 12-5 Chapter 12 Digestive System and Nutrition 217 The Esophagus The Wall of the Digestive Tract The esophagus is a muscular tube that passes from the pharynx through the thoracic cavity and diaphragm into the abdominal cavity where it joins the stomach. The esophagus is ordinarily collapsed, but it opens and receives the bolus when swallowing occurs. A rhythmic contraction called peristalsis pushes the food along the digestive tract. Occasionally, peristalsis begins even though there is no food in the esophagus. This produces the sensation of a lump in the throat. The esophagus plays no role in the chemical digestion of food. Its sole purpose is to conduct the food bolus from the mouth to the stomach. Sphincters are muscles that encircle tubes and act as valves; tubes close when sphincters contract, and they open when sphincters relax. The entrance of the esophagus to the stomach is marked by a constriction, often called a sphincter, although the muscle is not as developed as in a true sphincter. Relaxation of the sphincter allows the bolus to pass into the stomach, while contraction prevents the acidic contents of the stomach from backing up into the esophagus. Heartburn, which feels like a burning pain rising up into the throat, occurs when some of the stomach contents escape into the esophagus. When vomiting occurs, a contraction of the abdominal muscles and diaphragm propels the contents of the stomach upward through the esophagus. The wall of the esophagus in the abdominal cavity is comparable to that of the digestive tract, which has these layers (Fig. 12.4): The esophagus conducts the bolus of food from the pharynx to the stomach. Peristalsis begins in the esophagus and occurs along the entire length of the digestive tract. Mucosa (mucous membrane layer) A layer of epithelium supported by connective tissue and smooth muscle lines the lumen (central cavity) and contains glandular epithelial cells that secrete digestive enzymes and goblet cells that secrete mucus. Submucosa (submucosal layer) A broad band of loose connective tissue that contains blood vessels. Lymph nodules, called Peyer’s patches, are in the submucosa. Like the tonsils, they help protect us from disease. Muscularis (smooth muscle layer) Two layers of smooth muscle make up this section. The inner, circular layer encircles the gut; the outer, longitudinal layer lies in the same direction as the gut. Serosa (serous membrane layer) Most of the digestive tract has a serosa, a very thin, outermost layer of squamous epithelium supported by connective tissue. The serosa secretes a serous fluid that keeps the outer surface of the intestines moist so that the organs of the abdominal cavity slide against one another. The esophagus has an outer layer composed only of loose connective tissue called the adventitia. adventitia 2.5 mm lumen mucosa submucosa muscularis circular muscle longitudinal muscle serosa a. b. Figure 12.4 Wall of the digestive tract. a. Several different types of tissues are found in the wall of the digestive tract. Note the placement of circular muscle inside longitudinal muscle. b. Micrograph of the wall of the esophagus. Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 218 Part 3 12-6 Maintenance of the Human Body The Stomach The stomach (Fig. 12.5) is a thick-walled, J-shaped organ that lies on the left side of the body beneath the diaphragm. The stomach is continuous with the esophagus above and the duodenum of the small intestine below. The stomach stores food and aids in digestion. The wall of the stomach has deep folds, which disappear as the stomach fills to an approximate capacity of one liter. Its muscular wall churns, mixing the food with gastric juice. The term gastric always refers to the stomach. The columnar epithelial lining of the stomach has millions of gastric pits, which lead into gastric glands. The gastric glands produce gastric juice. Gastric juice contains an enzyme called pepsin, which digests protein, plus hydrochloric acid (HCl) and mucus. HCl causes the stomach to have a high acidity with a pH of about 2, and this is beneficial because it kills most bacteria present in food. Although HCl does not digest food, it does break down the connective tissue of meat and activates pepsin. The wall of the stomach is protected by a thick layer of mucus secreted by goblet cells in its lining. If, by chance, HCl penetrates this mucus, the wall can begin to break down, and an ulcer results. An ulcer is an open sore in the wall caused by the gradual disintegration of tissue. It now appears that most ulcers are due to a bacterial (Helicobacter pylori) infection that impairs the ability of epithelial cells to produce protective mucus. Alcohol is absorbed in the stomach, but there is no absorption of food substances. Normally, the stomach empties in about 2–6 hours. When food leaves the stomach, it is a thick, soupy liquid called chyme. Chyme leaves the stomach and enters the small intestine in squirts by way of a sphincter that repeatedly opens and closes. The stomach can expand to accommodate large amounts of food. When food is present, the stomach churns, mixing food with acidic gastric juice. gastric pit cells that secrete mucus gastric gland cells that secrete HCl and enzyme a. b. 20 µm stomach lining ulcer c. Figure 12.5 Anatomy and histology of the stomach. a. The stomach has a thick wall with folds that allow it to expand and fill with food. b. The mucosa contains gastric glands, which secrete mucus and a gastric juice active in protein digestion. c. View of a bleeding ulcer by using an endoscope (a tubular instrument bearing a tiny lens and a light source) that can be inserted into the abdominal cavity. Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 12-7 Chapter 12 The Small Intestine Digestive System and Nutrition 219 (Fig. 12.6). Each villus has an outer layer of columnar epithelium and contains blood vessels and a small lymphatic vessel called a lacteal. The lymphatic system is an adjunct to the cardiovascular system—its vessels carry a fluid called lymph to the cardiovascular veins. Each villus has thousands of microscopic extensions called microvilli. Collectively in electron micrographs, microvilli give the villi a fuzzy border known as a “brush border.” Since the microvilli bear the intestinal enzymes, these enzymes are called brush-border enzymes. The microvilli greatly increase the surface area of the villus for the absorption of nutrients. Sugars and amino acids pass through the mucosa and enter a blood vessel. The components of fats (glycerol and fatty acids) rejoin in smooth endoplasmic reticulum and are combined with proteins in the Golgi apparatus before they enter a lacteal. The small intestine is named for its small diameter (compared to that of the large intestine); but perhaps it should be called the long intestine. In life, the small intestine averages about 3 meters (9 feet) in length, compared to the large intestine which is about 1.5 meters (4 1⁄2 ft) in length. (After death, the small intestine becomes as long as 6 meters due to relaxation of muscles.) The first 25 cm of the small intestine is called the duodenum. Ducts from the liver and pancreas join to form usually one duct that enters the duodenum (see Fig. 12.1). The small intestine receives bile from the liver and pancreatic juice from the pancreas via this duct. Bile emulsifies fat—emulsification causes fat droplets to disperse in water. The intestine has a slightly basic pH because pancreatic juice contains sodium bicarbonate (NaHCO3), which neutralizes chyme. The enzymes in pancreatic juice and enzymes produced by the intestinal wall complete the process of digestion. It’s been suggested that the surface area of the small intestine is approximately that of a tennis court. What factors contribute to increasing its surface area? The wall of the small intestine contains fingerlike projections called villi, which give the intestinal wall a soft, velvety appearance The small intestine is specialized to absorb the products of digestion. It is quite long (3 meters) and has fingerlike projections called villi, where nutrient molecules are absorbed into the cardiovascular (glucose and amino acids) and lymphatic (fats) systems. 100 µm lumen villus Small intestine lacteal blood capillaries goblet cell lymph nodule arteriole venule Section of intestinal wall lymphatic vessel Villi Photomicrograph of villi Figure 12.6 Anatomy of small intestine. The wall of the small intestine has folds that bear fingerlike projections called villi. The products of digestion are absorbed by villi, which contain blood vessels and a lacteal. Each villus has many microscopic extensions called microvilli. Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 220 12-8 Maintenance of the Human Body Part 3 Regulation of Digestive Secretions The Large Intestine The nervous system promotes the secretion of digestive juices, but so do hormones (Fig. 12.7). A hormone is a substance produced by one set of cells that affects a different set of cells, the so-called target cells. Hormones are usually transported by the bloodstream. When a person has eaten a meal particularly rich in protein, the stomach produces the hormone gastrin. Gastrin enters the bloodstream, and soon the stomach is churning, and the secretory activity of gastric glands is increasing. A hormone produced by the duodenal wall, GIP (gastric inhibitory peptide), works opposite from gastrin: it inhibits gastric gland secretion. Cells of the duodenal wall produce two other hormones that are of particular interest—secretin and CCK (cholecystokinin). Acid, especially hydrochloric acid (HCl) present in chyme, stimulates the release of secretin, while partially digested protein and fat stimulate the release of CCK. Soon after these hormones enter the bloodstream, the pancreas increases its output of pancreatic juice, which helps digest food, and the liver increases its output of bile. The gallbladder contracts to release bile. The large intestine, which includes the cecum, the colon, the rectum, and the anal canal, is larger in diameter than the small intestine (6.5 cm compared to 2.5 cm), but it is shorter in length (1.5 meters compared to 3 meters) (see Fig. 12.1). The large intestine absorbs water, salts, and some vitamins. It also stores indigestible material until it is eliminated at the anus. The cecum, which lies below the junction with the small intestine, is the blind end of the large intestine. The cecum has a small projection called the vermiform appendix (vermiform means wormlike) (Fig. 12.8). In humans, the appendix also may play a role in fighting infections. This organ is subject to inflammation, a condition called appendicitis. If inflamed, the appendix should be removed before the fluid content rises to the point that the appendix bursts, a situation that may cause peritonitis, a generalized infection of the lining of the abdominal cavity. Peritonitis can lead to death. The colon includes the ascending colon, which goes up the right side of the body to the level of the liver; the transverse colon, which crosses the abdominal cavity just below the liver and the stomach; the descending colon, which passes down the left side of the body; and the sigmoid colon, which enters the rectum, the last 20 cm of the large intestine. The rectum opens at the anus, where defecation, the expulsion of feces, occurs. When feces are forced into the rectum by peristalsis, a defecation reflex occurs. The stretching of the rectal wall initiates nerve impulses to the spinal cord, and shortly thereafter contraction of the rectal muscles and relaxation of anal sphincters occur (Fig. 12.9). Feces are threequarters water and one-quarter solids. Bacteria, fiber (indigestible remains), and other indigestible materials are in the solid portion. The brown color of feces is due to bilirubin (see page 222), and the odor is due to breakdown products as bacteria work on the nondigested remains. This bacterial action also produces gases. For many years, it was believed that facultative bacteria (bacteria that can live with or without oxygen), such as Escherichia coli, were the major inhabitants of the colon, but new culture methods show that over 99% of the colon bacteria are obligate anaerobes (bacteria that die in the presence of oxygen). Not only do the bacteria break down indigestible material, they also produce some vitamins and other molecules that can be absorbed and used by us. In this way, they perform a service for us. Water is considered unsafe for swimming when the coliform (nonpathogenic intestinal) bacterial count reaches a certain number. A high count is an indication that a significant amount of feces has entered the water. The more feces present, the greater the possibility that disease-causing bacteria are also present. Gastrin 1 Secretin CCK 2 3 liver stomach gallbladder 2 3 pancreas 1 3 2 1 2 3 duodenum blood vessel Figure 12.7 Hormonal control of digestive gland secretions. Gastrin ➀ , produced by the lower part of the stomach, enters the bloodstream and thereafter stimulates the upper part of the stomach to produce more digestive juice. Secretin ➁ and CCK ➂ , produced by the duodenal wall, stimulate the pancreas to secrete its digestive juice and the gallbladder to release bile. Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 12-9 Chapter 12 Digestive System and Nutrition 221 spinal cord large intestine small intestine cecum motor nerve fibers sensory nerve fibers Stretch receptors initiate impulses to spinal cord. pelvic nerve Motor impulses cause contraction of rectal muscles and relaxation of anal sphincters. rectum internal anal sphincters appendix external anal sphincters Figure 12.8 Junction of the small intestine and the large Figure 12.9 Defecation reflex. intestine. The accumulation of feces in the rectum causes it to stretch, which initiates a reflex action resulting in rectal contraction and expulsion of the fecal material. The cecum is the blind end of the ascending colon. The appendix is attached to the cecum. Polyps The colon is subject to the development of polyps, small growths arising from the epithelial lining. Polyps, whether benign or cancerous, can be removed surgically. If colon cancer is detected while still confined to a polyp, the expected outcome is a complete cure. Some investigators believe that dietary fat increases the likelihood of colon cancer because dietary fat causes an increase in bile secretion. It could be that intestinal bacteria convert bile salts to substances that promote the development of cancer. On the other hand, fiber in the diet seems to inhibit the development of colon cancer. Dietary fiber absorbs water and adds bulk, thereby diluting the concentration of bile salts and facilitating the movement of substances through the intestine. Regular elimination reduces the time that the colon wall is exposed to any cancerpromoting agents in feces. Diarrhea and Constipation Two common everyday complaints associated with the large intestine are diarrhea and constipation. The major causes of diarrhea are infection of the lower tract and nervous stimulation. In the case of infection, such as food poisoning caused by eating contaminated food, the intestinal wall becomes irritated, and peristalsis increases. Water is not absorbed, and the diarrhea that results rids the body of the infectious organisms. In nervous diarrhea, the nervous system stimulates the intestinal wall, and diarrhea results. Pro- Back Forward Main Menu TOC Study Guide TOC longed diarrhea can lead to dehydration because of water loss and to disturbances in the heart’s contraction due to an imbalance of salts in the blood. When a person is constipated, the feces are dry and hard. One reason for this condition is that socialized persons have learned to inhibit defecation to the point that the desire to defecate is ignored. Two components of the diet that can help prevent constipation are water and fiber. Water intake prevents drying out of the feces, and fiber provides the bulk needed for elimination. The frequent use of laxatives is discouraged. If, however, it is necessary to take a laxative, a bulk laxative is the most natural because, like fiber, it produces a soft mass of cellulose in the colon. Lubricants, like mineral oil, make the colon slippery, and saline laxatives, like milk of magnesia, act osmotically—they prevent water from being absorbed and, depending on the dosage, may even cause water to enter the colon. Some laxatives are irritants; they increase peristalsis to the degree that the contents of the colon are expelled. Chronic constipation is associated with the development of hemorrhoids, enlarged and inflamed blood vessels at the anus. The large intestine does not produce digestive enzymes; it does absorb water, salts, and some vitamins. Textbook Website Student OLC MHHE Website 222 Part 3 12.2 12-10 Maintenance of the Human Body Three Accessory Organs The pancreas, liver, and gallbladder are accessory digestive organs. Figure 12.1 shows how the pancreatic duct from the pancreas and the common bile duct from the liver and gallbladder join before entering the duodenum. central vein sinusoid M hepatic portal vein The Pancreas The pancreas lies deep in the abdominal cavity, resting on the posterior abdominal wall. It is an elongated and somewhat flattened organ that has both an endocrine and an exocrine function. As an endocrine gland it secretes insulin and glucagon, hormones that help keep the blood glucose level within normal limits. We are now interested in its exocrine function. Most pancreatic cells produce pancreatic juice, which contains sodium bicarbonate (NaHCO3) and digestive enzymes for all types of food. Sodium bicarbonate neutralizes chyme; whereas pepsin acts best in an acid pH of the stomach, pancreatic enzymes require a slightly basic pH. Pancreatic amylase digests starch, trypsin digests protein, and lipase digests fat. In cystic fibrosis, a thick mucus blocks the pancreatic duct, and the patient must take supplemental pancreatic enzymes by mouth for proper digestion to occur. The liver, which is the largest organ in the body, lies mainly in the upper right section of the abdominal cavity, under the diaphragm (see Fig. 12.1). The liver has two main lobes, the right lobe and the smaller left lobe, which crosses the midline and lies above the stomach. The liver contains approximately 100,000 lobules that serve as the structural functional units of the liver (Fig. 12.10). Triads consisting of these three structures are located between the lobules: (1) a branch of the hepatic artery that brings oxygenated blood to the liver; (2) a branch of the hepatic portal vein that transports nutrients from the intestines; and (3) a bile duct that takes bile away from the liver. The central veins of lobules enter the hepatic vein. Note in Figure 12.11 that the liver lies between the hepatic portal vein (number 2 in the figure) and the hepatic vein (number 4 in the figure), which enters the vena cava. In some ways, the liver acts as the gatekeeper to the blood. As the blood from the intestines passes through the liver, it removes poisonous substances and works to keep the contents of the blood constant. It also removes and stores iron and the fat-soluble vitamins A, D, E, and K. The liver makes the plasma proteins from amino acids, and lipids from fatty acids. It also produces cholesterol and helps regulate the quantity of this substance in the blood. The liver maintains the blood glucose level at about 100 mg/100 ml (0.1%), even though a person eats intermittently. Any excess glucose that is present in the hepatic portal vein is removed and stored by the liver as glycogen. Be- Forward to hepatic vein bile duct hepatic artery from gut capillaries Figure 12.10 Hepatic lobules. The liver contains over 100,000 lobules. Each lobule contains many cells that perform the various functions of the liver. They remove from and/or add materials to blood and deposit bile in bile ducts. The Liver Back bile canals Main Menu TOC tween eating, glycogen is broken down to glucose, which enters the hepatic vein, and in this way, the blood glucose level remains constant. If the supply of glycogen is depleted, the liver will convert glycerol (from fats) and amino acids to glucose molecules. The conversion of amino acids to glucose necessitates deamination, the removal of amino acids. By a complex metabolic pathway, the liver then combines ammonia with carbon dioxide to form urea: O 2 NH3 ammonia + CO2 carbon dioxide H2N — C — NH2 urea Urea is the usual nitrogenous waste product from amino acid breakdown in humans. After its formation in the liver, urea is excreted by the kidneys. The liver produces bile, which is stored in the gallbladder. Bile has a yellowish green color because it contains the bile pigment bilirubin, derived from the breakdown of hemoglobin, the red pigment of red blood cells. Bile also contains bile salts, which are derived from cholesterol and emulsify fat in the small intestine. When fat is emulsified, it breaks Study Guide TOC Textbook Website Student OLC MHHE Website 12-11 Chapter 12 inferior vena cava 4. Blood enters general circulation by way of hepatic vein. 3. Liver monitors blood content. 2. Nutrient molecules travel in hepatic portal vein to liver. 1. Small intestine absorbs products of digestion. Figure 12.11 Hepatic portal system. The hepatic portal vein takes the products of digestion from the digestive system to the liver, where they are processed before entering the cardiovascular system proper. up into droplets, providing a much larger surface area, which can be acted upon by a digestive enzyme from the pancreas. Altogether, the following are significant functions of the liver: 1. Detoxifies blood by removing and metabolizing poisonous substances. 2. Stores iron (Fe2) and the fat-soluble vitamins A, D, E, and K. 3. Makes plasma proteins, such as albumins and fibrinogen, from amino acids. 4. Stores glucose as glycogen after eating, and breaks down glycogen to glucose to maintain the glucose concentration of blood between eating periods. 5. Produces urea from the breakdown of amino acids. 6. Removes bilirubin, a breakdown product of hemoglobin from the blood, and excretes it in bile, a liver product. 7. Produces lipids from fatty acids; produces and helps regulate blood cholesterol level, converting some to bile salts. Digestive System and Nutrition 223 Bilirubin is deposited in the skin due to an abnormally large amount in the blood. In hemolytic jaundice, red blood cells have been broken down in abnormally large amounts; in obstructive jaundice, bile ducts are blocked or liver cells are damaged. Jaundice can also result from hepatitis, inflammation of the liver. Viral hepatitis occurs in several forms. Hepatitis A is usually acquired from sewage-contaminated drinking water. Hepatitis B, which is usually spread by sexual contact, can also be spread by blood transfusions or contaminated needles. The hepatitis B virus is more contagious than the AIDS virus, which is spread in the same way. Thankfully, however, there is now a vaccine available for hepatitis B. Hepatitis C, which is usually acquired by contact with infected blood and for which there is no vaccine, can lead to chronic hepatitis, liver cancer, and death. Cirrhosis is another chronic disease of the liver. First the organ becomes fatty, and liver tissue is then replaced by inactive fibrous scar tissue. Cirrhosis of the liver is often seen in alcoholics due to malnutrition and to the excessive amounts of alcohol (a toxin) the liver is forced to break down. The liver has amazing generative powers and can recover if the rate of regeneration exceeds the rate of damage. During liver failure, however, there may not be enough time to let the liver heal itself. Liver transplantation is usually the preferred treatment for liver failure, but artificial livers have been developed and tried in a few cases. One type is a cartridge that contains liver cells. The patient’s blood passes through cellulose acetate tubing of the cartridge and is serviced in the same manner as with a normal liver. In the meantime, the patient’s liver has a chance to recover. The Gallbladder The gallbladder is a pear-shaped, muscular sac attached to the surface of the liver (see Fig. 12.1). About 1,000 ml of bile are produced by the liver each day, and any excess is stored in the gallbladder. Water is reabsorbed by the gallbladder so that bile becomes a thick, mucus-like material. When needed, bile leaves the gallbladder and proceeds to the duodenum via the common bile duct. The cholesterol content of bile can come out of solution and form crystals. If the crystals grow in size, they form gallstones. The passage of the stones from the gallbladder may block the common bile duct and cause obstructive jaundice. Then the gallbladder must be removed. Liver Disorders Jaundice, hepatitis, and cirrhosis are three serious diseases that affect the entire liver and hinder its ability to repair itself. Therefore, they are life-threatening diseases. When a person has jaundice, there is a yellowish tint to the whites of the eyes and also to the skin of light-pigmented persons. Back Forward Main Menu TOC Study Guide TOC The pancreas produces pancreatic juice, which contains enzymes for the digestion of food. Among its many functions, the liver produces bile, which is stored in the gallbladder. Textbook Website Student OLC MHHE Website 224 Part 3 12.3 12-12 Maintenance of the Human Body Lipase, a third pancreatic enzyme, digests fat molecules in the fat droplets after they have been emulsified by bile salts: Digestive Enzymes The digestive enzymes are hydrolytic enzymes, which break down substances by the introduction of water at specific bonds. Digestive enzymes, like other enzymes, are proteins with a particular shape that fits their substrate. They also have an optimum pH, which maintains their shape, thereby enabling them to speed up their specific reaction. The various digestive enzymes present in the digestive juices, mentioned previously, help break down carbohydrates, proteins, nucleic acids, and fats, the major components of food. Starch is a carbohydrate, and its digestion begins in the mouth. Saliva from the salivary glands has a neutral pH and contains salivary amylase, the first enzyme to act on starch: salivary amylase starch + → H2O maltose In this equation, salivary amylase is written above the arrow to indicate that it is neither a reactant nor a product in the reaction. It merely speeds the reaction in which its substrate, starch, is digested to many molecules of maltose, a disaccharide. Maltose molecules cannot be absorbed by the intestine; additional digestive action in the small intestine converts maltose to glucose, which can be absorbed. Protein digestion begins in the stomach. Gastric juice secreted by gastric glands has a very low pH—about 2—because it contains hydrochloric acid (HCl). Pepsinogen, a precursor that is converted to the enzyme pepsin when exposed to HCl, is also present in gastric juice. Pepsin acts on protein to produce peptides: pepsin protein + → H2O peptides Peptides vary in length, but they always consist of a number of linked amino acids. Peptides are usually too large to be absorbed by the intestinal lining, but later they are broken down to amino acids in the small intestine. Starch, proteins, nucleic acids, and fats are all enzymatically broken down in the small intestine. Pancreatic juice, which enters the duodenum, has a basic pH because it contains sodium bicarbonate (NaHCO3). Sodium bicarbonate neutralizes chyme, producing the slightly basic pH that is optimum for pancreatic enzymes. One pancreatic enzyme, pancreatic amylase, digests starch: bile salts → fat fat droplets lipase → fat droplets + H2O glycerol + fatty acids The end products of lipase digestion, glycerol and fatty acid molecules, are small enough to cross the cells of the intestinal villi, where absorption takes place. As mentioned previously, glycerol and fatty acids enter the cells of the villi, and within these cells, they are rejoined and packaged as lipoprotein droplets before entering the lacteals (see Fig. 12.6). Peptidases and maltase, two enzymes secreted by the small intestine, complete the digestion of protein to amino acids and starch to glucose, respectively. Amino acids and glucose are small molecules that cross into the cells of the villi. Peptides, which result from the first step in protein digestion, are digested to amino acids by peptidases: peptidases peptides + H2O → amino acids Maltose, a disaccharide that results from the first step in starch digestion, is digested to glucose by maltase: maltase maltose + H2O → glucose + glucose Other disaccharides, each of which has its own enzyme, are digested in the small intestine. The absence of any one of these enzymes can cause illness. For example, many people, including as many as 75% of African Americans, cannot digest lactose, the sugar found in milk, because they do not produce lactase, the enzyme that converts lactose to its components, glucose and galactose. Drinking untreated milk often gives these individuals the symptoms of lactose intolerance (diarrhea, gas, cramps), caused by a large quantity of nondigested lactose in the intestine. In most areas, it is possible to purchase milk made lactose-free by the addition of synthetic lactase or Lactobacillus acidophilus bacteria, which break down lactose. Table 12.2 lists some of the major digestive enzymes produced by the digestive tract, salivary glands, or the pancreas. Each type of food is broken down by specific enzymes. pancreatic amylase starch + → H2O maltose Another pancreatic enzyme, trypsin, digests protein: trypsin protein + H2O → peptides Trypsin is secreted as trypsinogen, which is converted to trypsin in the duodenum. Back Forward Main Menu TOC Study Guide TOC Digestive enzymes present in digestive juices help break down food to the nutrient molecules: glucose, amino acids, fatty acids, and glycerol. The first two are absorbed into the blood capillaries of the villi, and the last two re-form within epithelial cells before entering the lacteals as lipoprotein droplets. Textbook Website Student OLC MHHE Website 12-13 Chapter 12 Digestive System and Nutrition 225 Table 12.2 Major Digestive Enzymes Food Digestion Starch H2O → maltose Starch → glucose Protein H2O → peptides Maltose H2O Protein Peptides H2O → amino acids → nucleotides RNA and DNA H2O Enzyme Optimum pH Produced by Site of Action Salivary amylase Neutral Salivary glands Mouth Pancreatic amylase Basic Pancreas Small intestine Maltase Basic Small intestine Small intestine Pepsin Acidic Gastric glands Stomach Trypsin Basic Pancreas Small intestine Peptidases Basic Small intestine Small intestine Nuclease Basic Pancreas Small intestine Nucleosidases Basic Small intestine Small intestine Lipase Basic Pancreas Small intestine Nucleic Acid Nucleotides Fat → bases, sugars, phosphate ions Fat droplets H2O → glycerol + fatty acids Incubator 1 2 3 4 water pepsin water HCl water pepsin HCl water egg white egg white egg white egg white no digestion little or no digestion no digestion digestion Figure 12.12 Digestion experiment. This experiment is based on the optimum conditions for digestion by pepsin in the stomach. Knowing that the correct enzyme, optimum pH, optimum temperature, and the correct substrate must be present for digestion to occur, explain the results of this experiment. Conditions for Digestion Laboratory experiments can define the necessary conditions for digestion. For example, the four test tubes described in Figure 12.12 can be prepared and observed for the digestion of egg white, a protein digested in the stomach by the enzyme pepsin. After all tubes are placed in an incubator at body temperature for at least one hour, the results depicted are observed. Tube 1 is a control tube; no digestion has occurred in this tube because the enzyme and HCl are missing. (If a con- Back Forward Main Menu TOC Study Guide TOC trol gives a positive result, then the experiment is invalidated.) Tube 2 shows limited or no digestion because HCl is missing, and therefore the pH is too high for pepsin to be effective. Tube 3 shows no digestion because although HCl is present, the enzyme is missing. Tube 4 shows the best digestive action because the enzyme is present and the presence of HCl has resulted in an optimum pH. This experiment supports the hypothesis that for digestion to occur, the substrate and enzyme must be present and the environmental conditions must be optimum. The optimal environmental conditions include a warm temperature and the correct pH. Textbook Website Student OLC MHHE Website 226 Part 3 12.4 12-14 Maintenance of the Human Body Nutrition The body requires three major classes of macronutrients in the diet: carbohydrate, protein, and fat. These supply the energy and the building blocks that are needed to synthesize cellular contents. Micronutrients—especially vitamins and minerals—are also required because they are necessary for optimum cellular metabolism. Several modern nutritional studies suggest that certain nutrients can protect against heart disease, cancer, and other serious illnesses. These studies include an analysis of the eating habits of people in the United States and from around the world, especially those with and without heart disease and cancer. The result has been the dietary recommendations illustrated by a food pyramid (Fig. 12.13). The bulk of the diet should consist of bread, cereal, rice, and pasta as energy sources. Whole grains are preferred over those that have been milled because they contain fiber and vitamins and minerals. Vegetables and fruits are another rich source of fiber, vitamins, and minerals. Notice, then, that a largely vegetarian diet is recommended. A vege- Fats, Oils, Sweets (Use sparingly) tarian diet is not only healthy, it also makes sense in another way. As a rule of thumb, it is generally stated that only about 10 percent of the energy available in a food source is incorporated into the tissues of a consumer. This being the case, it can be reasoned that 100 lbs of grain could directly result in 10 human lbs, but if fed to cattle it would result in only one human lb. Therefore, a larger human population can be sustained on grain than on grain-consuming animals. Animal products, especially meat, need only be minimally included in the diet; fats and sweets should be used sparingly. Dairy products and meats tend to be high in saturated fats, and an intake of saturated fats increases the risk of cardiovascular disease. Low-fat dairy products are available, but there is no way to take much of the fat out of beef, which has a relatively high fat content. Ironically, the affluence of people in the United States contributes to a poor diet and, therefore, possible illness. Only comparatively rich people can afford fatty meats from grain-fed cattle and carbohydrates that have been highly processed to remove fiber and to add sugar and salt. Key Fat (naturally occurring and added) Sugars (added) These symbols show fat and added sugars in foods. Meat, Poultry, Fish, Dry Beans, Eggs, Nuts (2-3 servings) Milk, Yogurt, Cheese (2-3 servings) Vegetables (3-5 servings) Fruit (2-4 servings) Bread, Cereals, Rice, Pasta (6-11 servings) Figure 12.13 Food guide pyramid: A guide to daily food choices. The U.S. Department of Agriculture uses a pyramid to show the ideal diet because it emphasizes the importance of including grains, fruits, and vegetables in the diet. Meats and dairy products are needed in limited amounts; fats, oils, and sweets should be used sparingly. Source: Data from the U.S. Department of Agriculture. Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 12-15 Chapter 12 Carbohydrates The quickest, most readily available source of energy for the body is glucose. Carbohydrates are digested to simple sugars, which are or can be converted to glucose. Glucose is stored by the liver in the form of glycogen. Between eating periods, the blood glucose level is maintained at about 100 mg/100 ml of blood by the breakdown of glycogen or by the conversion of glycerol (from fats) or amino acids to glucose. If necessary, amino acids are taken from the muscles—even from the heart muscle. While body cells can utilize fatty acids as an energy source, brain cells require glucose. For this reason alone, it is necessary to include carbohydrates in the diet. According to Fig. 12.13, carbohydrates should make up the bulk of the diet. Further, these carbohydrates should be complex and not simple carbohydrates. Complex sources of carbohydrates include preferably whole-grain pasta, rice, bread, and cereal (Fig 12.14). Potatoes and corn, although considered vegetables, are also sources of carbohydrates. Simple carbohydrates (e.g., sugars) are labeled “empty calories” by some dieticians because they contribute to energy needs and weight gain without supplying any other nutritional requirements. Table 12.3 gives suggestions on how to reduce dietary sugars (simple carbohydrates). In contrast to simple sugars, complex carbohydrates are likely to be accompanied by a wide range of other nutrients and by fiber, which is indigestible plant material. The intake of fiber is recommended because it decreases the risk of colon cancer, a major type of cancer, and cardiovascular disease, the number one killer in the United States. Insoluble fiber, such as that found in wheat bran, has a laxative effect and may guard against colon cancer, because any Digestive System and Nutrition 227 cancer-causing substances are in contact with the intestinal wall for a limited amount of time. Soluble fiber, such as that found in oat bran, combines with bile acids and cholesterol in the intestine and prevents them from being absorbed. The liver now removes cholesterol from the blood and changes it to bile acids, replacing those that were lost. While the diet should have an adequate amount of fiber, a high-fiber diet can be detrimental. Some evidence suggests that the absorption of iron, zinc, and calcium is impaired by a diet too high in fiber. Complex carbohydrates, which contain fiber, should form the bulk of the diet. Table 12.3 Reducing Dietary Sugar To reduce dietary sugar: 1. Eat fewer sweets, such as candy, soft drinks, ice cream, and pastry. 2. Eat fresh fruits or fruits canned without heavy syrup. 3. Use less sugar—white, brown, or raw—and less honey and syrups. 4. Avoid sweetened breakfast cereals. 5. Eat less jelly, jam, preserves 6. Drink pure fruit juices, not imitations. 7. When cooking, use spices like cinnamon instead of sugar to flavor foods. 8. Do not put sugar in tea or coffee. Figure 12.14 Complex carbohydrates. To meet our energy needs, dieticians recommend consuming foods rich in complex carbohydrates, like those shown here, rather than foods consisting of simple carbohydrates, like candy and ice cream. Simple carbohydrates provide monosaccharides but few other types of nutrients. Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 228 Part 3 12-16 Maintenance of the Human Body Proteins Foods rich in protein include red meat, fish, poultry, dairy products, legumes (i.e., peas and beans), nuts, and cereals. Following digestion of protein, amino acids enter the bloodstream and are transported to the tissues. Ordinarily, amino acids are not used as an energy source. Most are incorporated into structural proteins found in muscles, skin, hair, and nails. Others are used to synthesize such proteins as hemoglobin, plasma proteins, enzymes, and hormones. Adequate protein formation requires 20 different types of amino acids. Of these, eight are required from the diet in adults (nine in children) because the body is unable to produce them. These are termed the essential amino acids. The body produces the other 11 amino acids by simply transforming one type into another type. Some protein sources, such as meat, are complete; they provide all 20 types of amino acids. Vegetables and grains supply us with amino acids, but each vegetable or grain alone is an incomplete protein source because of a deficiency in at least one of the essential amino acids. Absence of one essential amino acid prevents utilization of the other 19 amino acids. Soybeans and tofu, made from soybeans, are rich in amino acids, but it is wise to combine foods to acquire all the essential amino acids. For example, the combinations of cereal with milk, or beans, a legume, with rice, a grain, will provide all the essential amino acids (Table 12.4). Amino acids are not stored in the body, and a daily supply is needed. However, it does not take very much protein to meet the daily requirement. Two servings of meat a day (equal in total quantity to a deck of cards) is usually enough. Some meats (e.g., hamburger) are high in protein but also high in fat. Everything considered, it is probably a good idea to depend on protein from plant origins (e.g., whole-grain cereals, dark breads, and legumes) to a greater extent than is often the custom in the United States. This can be illustrated by the health statistics of native Hawaiians who no longer eat as their ancestors did (Fig. 12.15). The modern diet depends on animal rather than plant protein and is 42% fat. A statistical study showed that the island’s native peoples now have a higher than average death rate from cardiovascular disease and cancer. Diabetes is also common in persons who follow the modern diet. But the health of those who have switched back to the ancient diet has improved immensely! 10% 11% 73% 3% 3% Ancient 42% 28% 15% 12% 3% Modern KEY: complex carbohydrate animal protein fat sugar plant protein Figure 12.15 Ancient versus modern diet of native Hawaiians. Among those native Hawaiians who have switched back to the native diet, the incidence of cardiovascular disease, cancer, and diabetes has dropped. Table 12.4 Complementary Protein Combinations For any particular meal, choose foods from two or more columns. Legumes Grains Vegetables Seeds and Nuts Dried beans Barley Broccoli Nut butters Soy products Cornmeal Brussel sprouts Sesame seeds Dried lentils Pasta Leafy greens Cashews Peanuts Oats Sunflower seeds Dried peas Rice Walnuts Whole-grain breads Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 12-17 Chapter 12 Nutritionists do not recommend the use of protein and/or amino acid supplements. Protein supplements that people take to build muscle or as part of a diet are not digested as well as protein-rich foods and they cost more than food. Amino acid supplements can be dangerous to your health. An excess of any particular amino acid can lead to a deficiency of absorption of other amino acids present in lesser amounts. Contrary to popular reports, the taking of lysine does not relieve or cure herpes sores. It is also unwise to take tryptophan to relieve pain or cure insomnia. Some who have taken supplements of tryptophan have come down with a blood disorder (eosinophilia-myalgia syndrome) characterized by severe muscle and joint pain and swelling of the limbs. Digestive System and Nutrition 229 Table 12.5 Reducing Lipids To reduce dietary fat: 1. Choose poultry, fish, or dry beans and peas as a protein source. 2. Remove skin from poultry before cooking, and place on a rack so that fat drains off. 3. Broil, boil, or bake rather than fry. 4. Limit your intake of butter, cream, hydrogenated oils, shortenings, and tropical oils (coconut and palm oils).* 5. Use herbs and spices to season vegetables instead of butter, margarine, or sauces. Use lemon juice instead of salad dressing. 6. Drink skim milk instead of whole milk, and use skim milk in cooking and baking. 7. Eat nonfat or low-fat foods. Lipids To reduce dietary cholesterol: Fat and cholesterol are both lipids. Fat is present not only in butter, margarine, and oils, but also in many foods high in animal protein. The body can alter ingested fat to suit the body’s needs, except it is unable to produce linoleic acid, which is a polyunsaturated fatty acid. Saturated fatty acids have no double bonds; polyunsaturated fatty acids have many double bonds. The current guidelines suggest that fat should account for no more than 30% of our daily calories. The chief reason is that an intake of fat not only causes weight gain, it also increases the risk of cancer and cardiovascular disease. Dietary fat apparently increases the risk of colon, hepatic, and pancreatic cancers. Although recent studies suggest no link between dietary fat and breast cancer, other researchers still believe that the matter deserves further investigation. Cardiovascular disease is often due to arteries blocked by fatty deposits, called plaque, that contain saturated fats and cholesterol. Cholesterol is carried in the blood by two types of lipoproteins: low-density lipoprotein (LDL) and high-density lipoprotein (HDL). LDL is thought of as being “bad” because it carries cholesterol from the liver to the cells, while HDL is thought of as being “good” because it carries cholesterol to the liver, which takes it up and converts it to bile salts. Saturated fats, whether in butter or margarine, can raise LDL cholesterol levels, while monounsaturated (one double bond) fats and polyunsaturated (many double bonds) fats lower LDL cholesterol levels. Olive oil and canola oil contain mostly monounsaturated fats; corn oil and safflower oil contain mostly polyunsaturated fats. These oils have a liquid consistency and come from plants. Saturated fats, which are solids at room temperature, usually have an animal origin; two well-known exceptions are palm oil and coconut oil, which contain mostly saturated fats and come from the plants mentioned. Nutritionists stress that it is more important for the diet to be low in fat rather than be overly concerned about which Back Forward Main Menu TOC Study Guide TOC 1. Avoid cheese, egg yolks, liver, and certain shellfish (shrimp and lobster). Preferably, eat white fish and poultry. 2. Substitute egg whites for egg yolks in both cooking and eating. 3. Include soluble fiber in the diet. Oat bran, oatmeal, beans, corn, and fruits such as apples, citrus fruits, and cranberries are high in soluble fiber. *Although coconut and palm oils are from plant sources, they are mostly saturated fats. type fat is in the diet. Table 12.5 gives suggestions on how to reduce dietary fat. Fake Fat Olestra is a substance made to look, taste, and act like real fat but the digestive system is unable to digest it. Therefore it is commonly known as “fake fat.” Unfortunately, the fat-soluble vitamins A, D, E, and K tend to be taken up by olestra and thereafter they are not absorbed by the body. Similarly, people using olestra have reduced amounts of carotenoids in the blood. In one study, just a handful of olestra-soaked potato chips caused a 20% decline in blood beta-carotene levels. Manufacturers fortify olestra-containing foods with the vitamins mentioned but not carotenoids. More apparent, some people who consume olestra have developed anal leakage or underwear staining. Others experience diarrhea, intestinal cramping, and gas. Dietary protein supplies the essential amino acids; proteins from plant origins generally have less accompanying fat. A diet no more than 30% fat is recommended because fat intake, particularly saturated fats, are known to be associated with various health problems. Textbook Website Student OLC MHHE Website 230 Part 3 12-18 Maintenance of the Human Body Vitamins Vitamins are organic compounds (other than carbohydrate, fat, and protein) that the body is unable to produce but uses for metabolic purposes. Many vitamins are portions of coenzymes, which are enzyme helpers. For example, niacin is part of the coenzyme NAD, and riboflavin is part of another dehydrogenase, FAD. Coenzymes are needed in only small amounts because each can be used over and over again. Not all vitamins are coenzymes; vitamin A, for example, is a precursor for the visual pigment that prevents night blindness. If vitamins are lacking in the diet, various symptoms develop (Fig. 12.16). Altogether there are 13 vitamins, which are divided into those that are fat soluble (Table 12.6) and those that are water soluble (Table 12.7). Antioxidants Over the past 20 years, numerous statistical studies have been done to determine whether a diet rich in fruits and vegetables is protective against cancer. Cellular metabolism generates free radicals, unstable molecules that carry an extra electron. The most common free radicals in cells are the superoxide (O2) and hydroxide (OH). In order to stabilize themselves, free radicals donate an electron to DNA, or proteins, including enzymes, or lipids, which are found in plasma membranes. Such donations most likely damage these cellular molecules and thereby may lead to disorders, perhaps even cancer. Vitamins C, E, and A are believed to defend the body against free radicals, and therefore they are termed antioxidants. These vitamins are especially abundant in fruits and vegetables. The dietary guidelines shown in Figure 12.13 suggest that we eat a minimum of five servings of fruits and vegetables a day. To achieve this goal, include salad greens, raw or cooked vegetables, dried fruit, and fruit juice, in addition to traditional apples and oranges and such. Dietary supplements may provide a potential safeguard against cancer and cardiovascular disease, but nutritionists do not think it is appropriate to take supplements instead of improving intake of fruits and vegetables. There are many beneficial compounds in fruits that cannot be obtained from a vitamin pill. These compounds enhance each other’s absorption or action and also perform independent biological functions. Vitamin D Skin cells contain a precursor cholesterol molecule that first must be exposed to ultraviolet light and then when it enters the bloodstream must be acted on by the kidneys and liver before it finally becomes active vitamin D (calcitriol). Vitamin D promotes the absorption of calcium. Calcitriol promotes the absorption of calcium by the intestines. The lack of vitamin D leads to rickets in children (see Fig. 12.16a). Rickets, characterized by a bowing of the legs, is caused by defective mineralization of the skeleton. Most milk today is fortified with vitamin D, which helps prevent the occurrence of rickets. Vitamins are essential to cellular metabolism; many are protective against identifiable illnesses and conditions. c. b. a. Figure 12.16 Illnesses due to vitamin deficiency. a. Bowing of bones (rickets) due to vitamin D deficiency. b. Dermatitis (pellagra) of areas exposed to light due to niacin (vitamin B3) deficiency. c. Bleeding of gums (scurvy) due to vitamin C deficiency. Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 12-19 Chapter 12 Digestive System and Nutrition 231 Table 12.6 Fat-Soluble Vitamins Vitamin Functions Food Sources Conditions with Too Little Too Much Vitamin A Antioxidant synthesized from betacarotene; needed for healthy eyes, skin, hair, and mucous membranes, and for proper bone growth Deep yellow/orange vegetables, fruits, cheese, whole milk, butter, and eggs Night blindness, impaired growth of bones and teeth Headache, dizziness, nausea, hair loss Vitamin D A group of steroids needed for development and maintenance of bones and teeth Milk fortified with vitamin D, fish liver oil; also made in the skin when exposed to sunlight Rickets, bone decalcification and weakening Calcification of soft tissues, diarrhea, and possible renal damage Vitamin E Antioxidant that prevents oxidation of vitamin A and polyunsaturated fatty acids Leafy green vegetables, fruits, vegetable oils, nuts, wholegrain breads and cereals Unknown Diarrhea, nausea, headaches, fatigue, muscle weakness Vitamin K Needed for synthesis of substances active in clotting of blood Leafy green vegetables, cabbage and cauliflower Easy bruising and bleeding Can interfere with anticoagulant medication Table 12.7 Water-Soluble Vitamins Vitamin Back Functions Food Sources Conditions with Too Little Too Much Vitamin C Antioxidant; needed for forming collagen; helps maintain capillaries, bones, and teeth Citrus fruits, leafy green vegetables, tomatoes, potatoes, cabbage Scurvy, wounds heal slowly, infections Gout, kidney stones, diarrhea, decreased copper Thiamine (Vitamin B1) Part of coenzyme needed for cellular respiration; also promotes activity of the nervous system Whole-grain cereals, dried beans and peas, sunflower seeds, and nuts Beriberi, muscular weakness, heart enlarges Interferes with absorption of other vitamins Riboflavin (Vitamin B2) Part of coenzymes, such as FAD; aids cellular respiration, including oxidation of protein and fat Nuts, dairy products, wholegrain cereals, poultry, and leafy green vegetables Dermatitis, blurred vision, growth failure Unknown Niacin (Nicotinic acid) Part of coenzymes NAD and NADP; needed for cellular respiration, including oxidation of protein and fat Peanuts, poultry, wholegrain cereals, leafy green vegetables, and beans Pellagra, diarrhea, and mental disorders High blood sugar and uric acid, vasodilation, etc. Folacin (Folic acid) Coenzyme needed for production of hemoglobin and the formation of DNA Dark leafy green vegetables, nuts, beans, whole-grain cereals Megaloblastic anemia, spina bifida May mask B12 deficiency Vitamin B6 Coenzyme needed for the synthesis of hormones and hemoglobin; CNS control Whole-grain cereals, bananas, beans, poultry, nuts, leafy green vegetables Rarely, convulsions, vomiting, seborrhea, muscular weakness Insomnia Pantothenic acid Part of coenzyme A needed for oxidation of carbohydrates and fats; aids in the formation of hormones and certain neurotransmitters Nuts, beans, dark green vegetables, poultry, fruits, and milk Rarely, loss of appetite, mental depression, numbness Unknown Vitamin B12 Complex, cobalt-containing compound; part of the coenzyme needed for synthesis of nucleic acids and myelin Dairy products, fish, poultry, eggs, fortified cereals Pernicious anemia Unknown Biotin Coenzyme needed for metabolism of amino acids and fatty acids Generally in foods, especially eggs Skin rash, nausea, fatigue Unknown Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 232 12-20 Maintenance of the Human Body Part 3 Minerals In addition to vitamins, various minerals are required by the body. Minerals are divided into macrominerals and microminerals. The body contains more than 5 grams of each macromineral and less than 5 grams of each micromineral (Fig. 12.17). The macrominerals are constituents of cells and body fluids and are structural components of tissues. For example, calcium (present as Ca2) is needed for the construction of bones and teeth and for nerve conduction and muscle contraction. Phosphorus (present as PO43) is stored in the bones and teeth and is a part of phospholipids, ATP, and the nucleic acids. Potassium (K) is the major positive ion inside cells and is important in nerve conduction and muscle contraction, as is sodium (Na). Sodium also plays a major role in regulating the body’s water balance, as does chloride (Cl). Magnesium (Mg2) is critical to the functioning of hundreds of enzymes. The microminerals are parts of larger molecules. For example, iron is present in hemoglobin, and iodine is a part of thyroxin, a hormone produced by the thyroid gland. Zinc, copper, and selenium are present in enzymes that catalyze a variety of reactions. Proteins, called zinc-finger proteins because of their characteristic shapes, bind to DNA when a particular gene is to be activated. As research continues, more and more elements are added to the list of microminerals considered to be essential. During the past three decades, for example, very small amounts of selenium, molybdenum, chromium, nickel, vanadium, silicon, and even arsenic have been found to be essential to good health. Table 12.8 lists the functions of various minerals and gives their functions, food sources, and signs of deficiency and toxicity. Calcium Occasionally, individuals do not receive enough iron (especially women), calcium, magnesium, or zinc in their diet. Adult females need more iron in the diet than males (18 mg compared to 10 mg) because they lose hemoglobin each month during menstruation. Stress can bring on a magnesium deficiency, and due to its high-fiber content, a vegetarian diet may make zinc less available to the body. However, a varied and complete diet usually supplies enough of each type of mineral. Calcium Many people take calcium supplements (Fig. 12.17) to counteract osteoporosis, a degenerative bone disease that afflicts an estimated one-fourth of older men and one-half of older women in the United States. Osteoporosis develops because bone-eating cells called osteoclasts are more active than bone-forming cells called osteoblasts. Therefore, the bones are porous, and they break easily because they lack sufficient calcium. Due to recent studies that show consuming more calcium does slow bone loss in elderly people, the guidelines have been revised. A calcium intake of 1,000 mg a day is recommended for men and for women who are premenopausal or who use estrogen replacement therapy, and 1,300 mg a day is recommended for postmenopausal women who do not use estrogen replacement therapy. To achieve this amount, supplemental calcium is most likely necessary. Estrogen replacement therapy and exercise in addition to calcium supplements are effective means to prevent osteoporosis. Medications are also available that slow bone loss while increasing skeletal mass. Etidronate disodium needs to be taken cyclically or else weak, abnormal bone instead of normal bone develops. Fosamax is a new drug that can be taken continually but may accumulate in the skeleton. Therefore, studies are needed to determine if the drug can 1150 safely be given for a long time. 600 Phosphorus 210 Potassium 150 Sulfur Macrominerals 90 Sodium Presently, calcium supplements, estrogen therapy for women, and exercise are thought to be the best ways to prevent osteoporosis. 90 Chloride 30 Magnesium Iron 2.4 Manganese 0.18 Copper 0.09 Microminerals Iodine 0.024 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 Amount (g) Figure 12.17 Minerals in the body. This chart shows the usual amount of certain minerals in a 60-kilogram (135 lb) person. The macrominerals are present in amounts larger than 5 grams (about a teaspoon) and the microminerals are present in lesser amounts. The function of these minerals is given in Table 12.8. Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 12-21 Chapter 12 Digestive System and Nutrition 233 Table 12.8 Minerals Mineral Functions Food Sources Conditions with Too Little Too Much Macrominerals (more than 100 mg/day needed) Calcium (Ca2) Strong bones and teeth, nerve conduction, muscle contraction Dairy products, leafy green vegetables Stunted growth in children, low bone density in adults Kidney stones, interferes with iron and zinc absorption Phosphorus (PO43) Bone and soft tissue growth, part of phospholipids, ATP, and nucleic acids Meat, dairy products, sunflower seeds, food additives Weakness, confusion, pain in bones and joints Low blood and bone calcium levels Potassium (K) Nerve conduction, muscle contraction Many fruits and vegetables, bran Paralysis, irregular heart beat, eventual death Vomiting, heart attack, death Sodium (Na) Nerve conduction, pH and water balance Table salt Lethargy, muscle cramps, loss of appetite Edema, high blood pressure Chloride (Cl) Water balance Table salt Not likely Vomiting, dehydration Magnesium (Mg2) Part of various enzymes for nerve and muscle contraction, protein synthesis Whole grains, leafy green vegetables Muscle spasm, irregular heartbeat, convulsions, confusion, personality changes Diarrhea Microminerals (less than 20 mg/day needed) Zinc (Zn2) Protein synthesis, wound healing, fetal development and growth, immune function Meats, legumes, whole grains Delayed wound healing, night blindness, diarrhea, mental lethargy Anemia, diarrhea, vomiting, renal failure, abnormal cholesterol levels Iron (Fe2) Hemoglobin synthesis Whole grains, meats, prune juice Anemia, physical, and mental sluggishness Iron toxicity disease, organ failure, eventual death Copper (Cu2) Hemoglobin synthesis Meat, nuts, legumes Anemia, stunted growth in children Damage to internal organs if not excreted Iodine (I) Thyroid hormone synthesis Iodized table salt, seafood Thyroid deficiency Depressed thyroid, function, anxiety Selenium (SeO42) Part of antioxidant enzyme Seafood, meats, eggs Possible cancer development Hair and fingernail loss, discolored skin Sodium The recommended amount of sodium intake per day is 500 mg, although the average American takes in 4,000–4,700 mg every day. In recent years, this imbalance has caused concern because high sodium intake has been linked to hypertension (high blood pressure) in some people. About one-third of the sodium we consume occurs naturally in foods; another one-third is added during commercial processing; and we add the last one-third either during home cooking or at the table in the form of table salt. Clearly, it is possible for us to cut down on the amount of sodium in the diet. Table 12.9 gives recommendations for doing so. Table 12.9 Reducing Dietary Sodium To reduce dietary sodium: 1. Use spices instead of salt to flavor foods. 2. Add little or no salt to foods at the table, and add only small amounts of salt when you cook. 3. Eat unsalted crackers, pretzels, potato chips, nuts, and popcorn. 4. Avoid hot dogs, ham, bacon, luncheon meats, smoked salmon, sardines, and anchovies. 5. Avoid processed cheese and canned or dehydrated soups. 6. Avoid brine-soaked foods, such as pickles or olives. 7. Read labels to avoid high salt products. Excess sodium in the diet can lead to hypertension; therefore, excess sodium intake should be avoided. Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 234 12-22 Maintenance of the Human Body Part 3 Persons with obesity have • weight 20% or more above appropriate weight for height. • body fat content in excess of that consistent with optimal health; probably due to a diet rich in fats. • low levels of exercise. Figure 12.18 Recognizing obesity. Eating Disorders Authorities recognize three primary eating disorders: obesity, bulimia nervosa, and anorexia nervosa. Although they exist in a continuum as far as body weight is concerned, they all represent an inability to maintain normal body weight because of eating habits. Obesity As indicated in Figure 12.18, obesity is most often defined as a body weight 20% or more above the ideal weight for a person’s height. By this standard, 28% of women and 10% of men in the United States are obese. Moderate obesity is 41–100% above ideal weight, and severe obesity is 100% or more above ideal weight. Obesity is most likely caused by a combination of factors, including genetic, hormonal, metabolic, and social factors. It’s known that obese individuals have more fat cells than normal, and when they lose weight the fat cells simply get smaller; they don’t disappear. The social factors that cause obesity include the eating habits of other family members. Consistently eating fatty foods, for example, will make you gain weight. Sedentary activities, like watching television instead of exercising, also determine how much body fat you have. The risk of heart disease is higher in obese individuals, and this alone tells us that excess body fat is not consistent with optimal health. The treatment depends on the degree of obesity. Surgery to remove body fat may be required for those who are moderately or greatly overweight. For most, a knowledge of good eating habits along with behavior modification may suffice, particularly if a balanced diet is accompanied by a sensible exercise program. A lifelong commitment to a properly planned program is the best way to prevent a cycle of Back Forward Main Menu TOC weight gain followed by weight loss. Such a cycle is not conducive to good health. The Health reading on page 236 discusses the proper way to lose weight. Bulimia Nervosa Bulimia nervosa can coexist with either obesity or anorexia nervosa, which is discussed next. People with this condition have the habit of eating to excess (called binge eating) and then purging themselves by some artificial means, such as self-induced vomiting or use of a laxative. Bulimic individuals are overconcerned about their body shape and weight, and therefore they may be on a very restrictive diet. A restrictive diet may bring on the desire to binge, and typically the person chooses to consume sweets, like cakes, cookies, and ice cream (Fig. 12.19). The amount of food consumed is far beyond the normal number of calories for one meal, and the person keeps on eating until every bit is gone. Then, a feeling of guilt most likely brings on the next phase, which is a purging of all the calories that have been taken in. Bulimia can be dangerous to your health. Blood composition is altered, leading to an abnormal heart rhythm, and damage to the kidneys can even result in death. At the very least, vomiting may result in inflammation of the pharynx and esophagus, and stomach acids can cause teeth to erode. The esophagus and stomach may even rupture and tear due to strong contractions during vomiting. The most important aspect of treatment is to get the patient on a sensible and consistent diet. Again, behavioral modification is helpful and so perhaps is psychotherapy to help the patient understand the emotional causes of the behavior. Medications, including antidepressant medications, have sometimes been helpful to reduce the bulimic cycle and restore normal appetite. Study Guide TOC Textbook Website Student OLC MHHE Website 12-23 Chapter 12 Digestive System and Nutrition 235 Anorexia Nervosa Persons with bulimia nervosa have • recurrent episodes of binge eating characterized by consuming an amount of food much higher than normal for one sitting and a sense of lack of control over eating during the episode. • an obsession about their body shape and weight. Body weight is regulated by • a restrictive diet, excessive exercise. • purging (self-induced vomiting or misuse of laxatives). Figure 12.19 Recognizing bulimia nervosa. Persons with anorexia nervosa have • a morbid fear of gaining weight; body weight no more than 85% normal. • a distorted body image so that person feels fat even when emaciated. • in females, an absence of a menstrual cycle for at least three months. Body weight is kept too low by either/or In anorexia nervosa, a morbid fear of gaining weight causes the person to be on a very restrictive diet. Athletes such as distance runners, wrestlers, and dancers are at risk of anorexia nervosa because they believe that being thin gives them a competitive edge. In addition to eating only lowcalorie foods, the person may induce vomiting and use laxatives to bring about further loss of weight. No matter how thin they have become, people with anorexia nervosa think they are overweight (Fig. 12.20). Such a distorted self-image may prevent recognition of the need for medical help. Actually, the person is starving and has all the symptoms of starvation, such as low blood pressure, irregular heartbeat, constipation, and constant chilliness. Bone density decreases and stress fractures occur. The body begins to shut down; menstruation ceases in females; the internal organs including the brain don’t function well, and the skin dries up. Impairment of the pancreas and digestive tract means that any food consumed does not provide nourishment. Death may be imminent. If so, the only recourse may be hospitalization and force-feeding. Eventually, it is necessary to use behavior therapy and psychotherapy to enlist the cooperation of the person to eat properly. Family therapy may be necessary, because anorexia nervosa in children and teens is believed to be a way for them to gain some control over their lives. • a restrictive diet often with excessive exercise • binge eating/purging (person engages in binge eating and then self-induces vomiting or misuse of laxatives) In anorexia nervosa, the individual has a distorted body image and always feels fat. Medical help is often a necessity. Figure 12.20 Recognizing anorexia nervosa. Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website Weight Loss the Healthy Way Those who need to lose weight should reduce their caloric intake and/or increase their level of exercise. However, a diet needs to be judged according to the principles of adequacy of nutrients; balance in regard to carbohydrates, proteins, and fats; caloric content; and variety of food sources. Unfortunately, many of the diets and gimmicks people use to lose weight are bad for their health. Unhealthy approaches include the following: Pills The most familiar pills, and the only ones approved by the FDA, are those that claim to suppress the appetite. They may work at first, but the appetite soon returns to normal and weight lost is regained. Then the user has the problem of trying to get off the drug without gaining more weight. Other types of pills are under investigation and sometimes can be obtained illegally. But, as yet, there is no known drug that is both safe and effective for weight loss. Low-Carbohydrate Diets The dramatic weight loss that occurs with a low-carbohydrate diet is not due to a loss of fat; it is due to a loss of muscle mass and water. Glycogen and important minerals are also lost. When a normal diet is resumed, so is the normal weight. Liquid Diets Despite the fact that liquid diets provide proteins and vitamins, the number of calories is so restricted that the body cannot burn fat quickly enough to compensate, and muscle is still broken down to provide energy. A few people on this regime have died, probably because even the heart muscle was not spared by the body. Single-Category Diets These diets rely on the intake of only one kind of food, either a fruit or vegetable or rice alone. However, no single type of food provides the balance of nutrients needed to maintain health. Some dieters on strange diets suffer the consequences—in one instance, an individual lost hair and fingernails. Go To Student OLC Questions to Ask About a Weight-Loss Diet 1. Does the diet have a reasonable number of calories (Cal*), that is, usually no fewer than 1,000–1,200 Cal? 2. Does the diet provide the right amount of protein? For a 154 lb man, 56 grams is recommended. More than twice this amount is too much. For reference, 1c milk and 1 oz meat each has 8 grams protein. 3. Does the diet provide too much fat? No more than 20–30% of total Cal is recommended. For reference , a pat of butter has 45 Cal. 4. Does the diet provide enough carbohydrates? 100 grams 400 Cal is the very least recommended per day. For reference, a slice of bread contains 14 grams of carbohydrate. 5. Does the diet provide a balanced assortment of foods? 6. Does the diet make use of ordinary foods that are available locally? *Cal 1,000 calories A fat-free fat, called olestra, is now being used to produce foods that are free of calories from fat. A slice of pie ordinarily contains 405 calories, but when the crust is made with olestra, it has only 252 calories. Just three chocolate chip cookies ordinarily adds 63 calories to your daily calorie count, but with olestra it’s only 38 calories. And one brownie suddenly goes down from 85 to 49 calories. Olestra molecules made of six or eight fatty acids attached to a sugar molecule are much bigger than a triglyceride, and lipase can’t break the molecule down. Olestra goes through the intestines without being absorbed. Proctor and Gamble, who developed olestra, expects to make one billion dollars within ten years on its product. It is generally recognized that olestra can cause intestinal cramping, flatulence (gas), and diarrhea in a small segment of the population, and can prevent the absorption of some carotenoids, a nutrient that has health benefits in all of us. Even so, the Federal Drug Administration (FDA) approved it for use on the basis that olestra was reasonably harmless. Henry Blackburn, professor of public health at the University of Minnesota, thinks this standard is too low. Instead, the standard for approval ought to be, “Does this product contribute to the nutritional health of the nation?” David Kessler, FDA commissioner, says, “Ask the American people if government should be off the backs of business, and you will get a resounding Yes!” However, what is the role of government in protecting public health? Do Americans want more protection than they already have from food additives? Or would they rather be free to make their own choices? Questions 1. Should the government only approve food additives that will contribute to our health? Why or why not? 2. Are people responsible, themselves, for what they eat? Why or why not? 3. Is there too much emphasis in our culture on staying slim, despite what it may do to our health? Why or why not? 236 Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website 12-25 Chapter 12 Summarizing the Concepts 12.1 The Digestive System The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue forms a bolus for swallowing. The air passage and food passage cross in the pharynx. When one swallows, the air passage is usually blocked off and food must enter the esophagus where peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns, mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike projections called villi where nutrient molecules are absorbed into the cardiovascular and lymphatic systems. The large intestine consists of the cecum, colons (ascending, transverse, descending, and sigmoid), and the rectum, which ends at the anus. The large intestine does not produce digestive enzymes; it does absorb water, salts, and some vitamins. 12.2 Three Accessory Organs The three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It has numerous important functions, and any malfunction of the liver is a matter of considerable concern. 12.3 Digestive Enzymes Digestive enzymes are present in digestive juices and break down food into the nutrient molecules glucose, amino acids, fatty acids, and glycerol (see Table 12.2). Glucose and amino acids are absorbed into the blood capillaries of the villi. Fatty acids and glycerol rejoin to produce fat, which enters the lacteals. Digestive enzymes have the usual enzymatic properties. They are specific to their substrate and speed up specific reactions at body temperature and optimum pH. 12.4 Nutrition The nutrients released by the digestive process should provide us with an adequate amount of energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The bulk of the diet should be carbohydrates (like bread, pasta, and rice) and fruits and vegetables. These are low in saturated fatty acids and cholesterol molecules, whose intake is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cell contents from damage due to free radicals. Studying the Concepts 1. List the organs of the digestive tract, and state the contribution of each to the digestive process. 214–20 2. Discuss the absorption of the products of digestion into the lymphatic and cardiovascular systems. 219 3. Name and state the functions of the hormones that assist the nervous system in regulating digestive secretions. 220 4. Name the accessory organs, and describe the part they play in the digestion of food. 222–23 5. Choose and discuss any three functions of the liver. 222–23 6. Name and discuss three serious illnesses of the liver. 223 7. Discuss the digestion of starch, protein, and fat, listing all the steps that occur to bring about digestion of each of these. 224–25 Back Forward Main Menu TOC Study Guide TOC Digestive System and Nutrition 237 8. What is the chief contribution of each of these constituents of the diet: a. carbohydrates; b. proteins; c. fats; d. fruits and vegetables? 228–32 9. Why should the amount of saturated fat be curtailed in the diet? 230–32 10. Name and discuss three eating disorders. 234–35 Testing Yourself Choose the best answer for each question. 1. If you were tracing the path of food, which is out of order first? a. mouth d. small intestine b. pharynx e. stomach c. esophagus f. large intestine 2. Which association is incorrect? a. mouth—starch digestion b. esophagus—protein digestion c. small intestine—starch, lipid, protein digestion d. stomach—food storage e. liver—production of bile 3. Which of these could be absorbed directly without need of digestion? a. glucose d. protein b. fat e. nucleic acid c. polysaccharides 4. Which association is incorrect? a. protein—trypsin d. maltose—pepsin b. fat—bile e. starch—amylase c. fat—lipase 5. Most of the products of digestion are absorbed across the a. squamous epithelium of the esophagus. b. striated walls of the trachea. c. convoluted walls of the stomach. d. fingerlike villi of the small intestine. e. smooth wall of the large intestine. 6. Bile a. is an important enzyme for the digestion of fats. b. cannot be stored. c. is made by the gallbladder. d. emulsifies fat. e. All of these are correct. 7. Which of these is not a function of the liver in adults? a. produce bile d. produce urea b. detoxify alcohol e. make red blood cells c. store glucose 8. The large intestine a. digests all types of food. b. is the longest part of the intestinal tract. c. absorbs water. d. is connected to the stomach. e. is subject to hepatitis. 9. Bulimia nervosa and anorexia nervosa have many elements in common, except a. restrictive diet often with excessive exercise. b. binge eating followed by purging. c. obsession about body shape and weight. d. distorted body image so person feels fat even when emaciated. e. health can be damaged by this complex. Textbook Website Student OLC MHHE Website 238 Part 3 12-26 Maintenance of the Human Body 10. Label each organ indicated in the diagram. For the arrows, use either glucose, amino acids, lipids, or water. a. b. Digestion d. c. e. Absorption f. i. j. lymphatic vessel (to vein) g. h. k. feces anus blood vessel (to liver) Defecation Thinking Scientifically 1. Considering homeostasis: a. What type of tissue found in the mucous membrane of the stomach and the small intestine produces digestive enzymes? (page 217) b. All systems of the body contribute to homeostasis. How does the digestive system contribute? c. The liver “fine tunes” the contribution to homeostasis by the digestive system. How? 2. Considering the experiment described in Figure 12.12 (page 225): a. Does the experiment show that the enzyme pepsin prefers an acidic rather than a basic pH? Describe an experiment that would show this. b. Does the experiment described in Figure 12.12 show that pepsin will digest only protein and not starch, for example? Describe an experiment that would show this. c. Does the experiment described in Figure 12.12 show that better digestion occurs at a warm temperature rather than a cold temperature? Describe an experiment that would show this. glottis 216 hard palate 214 heartburn 217 hepatitis 223 hormone 220 hydrolytic enzyme 224 jaundice 223 lacteal 219 lactose intolerance 224 large intestine 220 lipase 222 liver 222 lumen 217 maltase 224 mineral 232 nasopharynx 216 obesity 234 pancreas 222 pancreatic amylase 222 pepsin 218 peptidase 224 periodontitis 215 peristalsis 217 peritonitis 220 pharynx 216 plaque 229 polyp 221 reflex action 216 salivary amylase 224 salivary gland 214 small intestine 219 soft palate 214 sphincter 217 stomach 218 tonsillitis 214 trypsin 222 ulcer 218 villus 219 vitamin 230 Match the terms to these definitions: a. First portion of the small intestine into which secretions from the liver and pancreas enter. b. Muscle that surrounds a tube and closes or opens the tube by contracting and relaxing. c. Discharge of feces from the rectum through the anus. d. Fat-digesting enzyme secreted by the pancreas. e. Saclike organ associated with the liver that stores and concentrates bile. Using Technology Your study of the digestive system and nutrition is supported by these available technologies: Essential Study Partner CD-ROM Animals £ Digestion Visit the Mader web site for related ESP activities. http://www.mhhe.com/biosci/genbio/mader/ (Click on Inquiry into Life.) Exploring the Internet The Mader Home Page provides resources and tools as you study this chapter. Understanding the Terms anorexia nervosa 235 anus 220 appendix 220 bile 219 bulimia nervosa 234 cecum 220 chyme 218 cirrhosis 223 colon 220 constipation 221 Back Forward Main Menu defecation 220 dental caries 215 diarrhea 221 duodenum 219 epiglottis 216 esophagus 217 essential amino acids 228 fiber 227 gallbladder 223 gastric gland 218 TOC Study Guide TOC http://www.mhhe.com/biosci/genbio/mader Dynamic Human 2.0 CD-ROM Human Body £ Digestive System Virtual Physiology Laboratory CD-ROM Digestion of Fat Enzyme Characteristics Textbook Website Student OLC MHHE Website